Rim drive electrical machine

ABSTRACT

A rim driven electrical machine for interaction with a flow of water, comprising: a rotor mounted within a housing, the rotor including at least one circumferential conductor and a central passageway, the central passageway for receiving at least one blade arrangement for rotational interaction with a fluid within the central passageway, and; a stator supported by the housing and arranged circumferentially around the rotor, the stator including windings for providing a travelling electromagnetic wave for interaction with the at least one circumferential electrical conductor of the rotor, wherein the stator extends only partially around the circumference of the rotor so as to have a first end and a second end separated by an arcuate angle.

This invention relates to rim drive electrical machines. In particular, this invention relates to rim drive electrical machines for sub-aqua applications.

Rim drive electrical machines are known. GB2440400 describes a rim drive marine propulsion unit which includes an impeller having an array of blades fastened between a hub and a rim which constitutes the rotor of an electrical motor and comprises a rotor core disposed within a stator of the motor. The stator has a circumferentially distributed series of windings which generate a rotating magnetic field for interaction with the permanent magnets and synchronous rotation of the rotor.

This invention seeks to provide an improved rim drive electrical machine.

Accordingly, the present invention provides a rim driven electrical machine for interaction with a flow of water, comprising: a rotor mounted within a housing, the rotor including at least one circumferential conductor and a central passageway, the central passageway for receiving at least one blade arrangement for rotational interaction with a fluid within the central passageway; and, a stator supported by the housing and arranged circumferentially around the rotor, the stator including windings for providing a travelling electromagnetic wave for interaction with the at least one circumferential electrical conductor of the rotor, wherein the stator extends only partially around the circumference of the rotor so as to have a first end and a second end separated by an arcuate angle.

Rim drive electrical machines generally have large diameter rotors which results in the machines having high torque densities for machines with short axial lengths. Having a stator which extends only partially around the rotor allows the axial length of the machine to be extended whilst maintaining the torque production in the machine (or generating capability in the case of a generator) and keeping the weight and associated cost relatively low.

Extending the axial length of the machine can be advantageous for sub-aqua applications which use a bladed arrangement for drive purposes. By drive, it will be understood that the rotor can be driven by the fluid flow through the rotor, or used to drive, i.e. propel, fluid through the rotor.

The rotor can include permanent magnets. Preferably, the rim drive electrical machine is an induction machine.

The machine may be a motor. Alternatively, the machine may be a generator.

There may be a plurality of stators. The stators may be arranged symmetrically around the circumference of the rotor.

Preferably, the ratio of stators to housing is less than 1:2.

The rotor may comprise an annular core of ferromagnetic material having external circumferential layers. The layers may comprise an environmental enclosure and the circumferential conductor. The at least one circumferential electrical conductor can be disposed between the annular core and the environmental enclosure.

The annular core can be provided with circumferentially distributed electrically conductive rotor bars. The annular core can be provided with circumferentially distributed permanent magnets.

The environmental enclosure can be made from an electrically conductive material.

The electrical resistance of the environmental enclosure can be higher than that of the or each circumferential electrical conductor.

The at least one circumferential electrical conductor can include apertures which define circumferentially distributed bars. The bars can extend substantially in the direction of the axis of rotation of the rotor.

The or each circumferential conductor can be a sleeve.

The rim drive electrical machine can further comprise at least two circumferential electrical conductors.

The layers in the rotor can have progressively reducing electrical resistance in the radially inwards direction.

The electrically conductive sleeve immediately adjacent the environmental enclosure can be radially thicker than the environmental enclosure.

The rotor can comprise an impeller or turbine having an array of blades secured to and extending inwardly of the annular core.

At least a portion of the housing and rotor can be separated by a substantially uniform gap which acts as a bearing surface far the rotor.

The stator may be located adjacent the lowest part of rotor when in use.

The rim drive electrical machine can be a marine propulsion unit.

The rim drive electrical machine can be a turbine. The turbine can be for tidal generation.

An embodiment of the invention will now be described with aid of the following drawings in which:

FIG. 1 shows a perspective view an electrical induction machine of a first embodiment of the invention.

FIG. 2 shows a propulsion unit of a second embodiment.

FIG. 3 shows an exploded view of an electrical induction machine of the second embodiment of the invention.

Thus, in FIG. 1 there is shown a rim drive electrical machine 10 in the form of an electrical induction machine. The electrical induction machine 10 includes a housing in which a rotor is located 14. The housing includes and thereby supports two stators 16 which are circumferentially arranged around the rotor 14, each including a plurality of windings 18 for providing a travelling electromagnetic wave which interacts with the rotor 14 so as to provide a torque on the rotor 14 when used as a motor. The stators 16 symmetrically oppose each other and extend only partially around the circumference of the rotor 14 so as to have a first end and a second end separated by an angle, thereby forming an arc. The remainder of the housing 12 simply forms a support structure for the stators 16 and rotor 14.

Generally, the torque developed by an induction machine is determined to a large extent by the magnetic and electric loadings and by the volume of the rotor. The magnetic and electric loadings are limited by material constraints in the form of magnetic saturation and winding insulation therefore high-torque induction machines normally have large diameters. Large diameter induction machines with short axial lengths make them ideally suited to rim drive propulsion motors with the motor integrated around the outer ‘rim’ of the propeller. The high diameter means that the machine can develop a high torque using normal magnetic and electric loadings so that the axial length can be limited to meet the required torque demand. The present invention provides a rim drive in which the energy density within the coils can be high whilst the axial length of the machine is made longer to suit a given application. This form of construction allows the machine to be cheaper to manufacture and relatively light as the remainder of the housing can made from lighter and preferably less expensive materials than the coils. Having a plurality of stators 16 allows the torque exerted on the rotor 14 to be balanced.

The rotor 14 includes several layers in the form of an outer environmental enclosure 20, a circumferential conductor in the form of a sleeve 22, an annular core 24 and a central passageway 26 which passes through the centre of the annular core 24 along the longitudinal axis of the rotor 14. The central passageway 26 provides a conduit for receiving at least one rotatably mounted blade arrangement (as can be seen in FIG. 2) such that a fluid flow through the passageway rotationally interacts with the blade arrangement. This rotational interaction can be used to extract or impart energy to the fluid flow thereby providing a generator or a propulsion unit, respectively.

The portion of the housing 12 not occupied by the stator 16 incorporates a light weight material which can provide structural support to the other constituent parts of the electrical machine.

Although the layers 20, 22 are shown in FIG. 1 as projecting one from the other, this is for purposes of illustration only; in practice the layers 20, 22 and annular core 24 terminate at each end so that the annular core 24 and circumferential conductor 22 are fully enclosed by the environmental enclosure 20.

The environmental enclosure 20 protects the circumferential conductor sleeve 22 and annular core 24 from the surroundings which, when the electromagnetic machine is used in a marine based application, such as a propulsion unit for a vessel or a tidal turbine, would be sea water. The stator 16 is protected from the operating environment by an encapsulation layer 28. The stator windings 18 are wound around pole pieces which are distributed around the circumferential arc of the stator 18 and are directed inwardly. There are two stators 16 in the embodiment shown in FIG. 1, each having three windings 18, but the skilled person will appreciate that the number of stators, length of the arc and the number of poles can be adjusted to suit a particular application.

The encapsulation layer 28 of the stator 16 and the environmental enclosure 20 of the rotor are separated by a substantially uniform gap. In a sub-aqua application this gap would be filled with water. Hence, the encapsulation layer provides a bearing surface in which the rotor 14 can rotate. This is particularly advantageous when using sleeves for the circumferential conductor 22 and environmental enclosures 20 as the electromagnetic separation between the stator 16 and rotor 14 can be kept to a minimum.

The circumferential conductor 22 and environmental enclosure 20 may be made from any suitable material which provides the necessary electrical performance and the environmental shielding required by the specific application. For example, the environmental enclosure 20 may be electrically conductive but is not necessarily so. It may, for example, be made from a composite material. An important function of the environmental enclosure 20 is to prevent penetration of sea water from the surroundings to the annular core 24. Consequently, its material is preferably selected to resist corrosion and to provide a good seal with adjacent components. The environmental enclosure 20 is a non-magnetic material. The environmental enclosure is preferably stainless steel.

The circumferential conductive 18 needs to be electrically conductive, and may be made from a metal or metal alloy such as steel or copper.

The annular core 24 may be a simple homogenous hollow cylinder, as represented in FIG. 1, but in other embodiments it may be provided with longitudinally extending grooves (possibly with a helical twist to them) for receiving conductors, such as cooper bars, which may be short-circuited (i.e. electrically interconnected) at the opposite axial ends of the core 10. Furthermore, the annular core 24 may be provided with circumferentially distributed permanent magnets so as to provide a synchronous machine.

In a preferred embodiment, the environmental enclosure 20 is electrically conductive, but its resistance is greater than that of the conductive sleeve. It will be appreciated that the resistance of each layer will be a function of its resistivity, thickness and length. Consequently, a desired electrical resistance may be achieved, for example, by appropriate selection of the materials from which the sleeves is made or the geometry chosen. These aspects are further described below with reference to the second embodiment which includes multiple sleeves.

The circumferential conductor sleeve 22 may be provided with apertures or slots (not shown in the Figures) which give the sleeve 22 a configuration which, in operation, achieves a desired distribution of rotor electrical currents. For example, the sleeve 22 could have a configuration similar to that of a squirrel cage (i.e. with axial end rings interconnected with longitudinally extending bars) which would cooperate with the annular core 24 to provide a squirrel-cage rotor effect. In another embodiment, the circumferential conductor may include conductive bars so as to form a traditional squirrel cage motor or generator.

In operation of an induction motor of the kind shown in the Figures, current is supplied to the windings 18 of the stator 16 in a controlled manner to generate a travelling magnetic field within the stator 14. The travelling magnetic field induces current flow within the opposing portion of the circumferential conductor 20 of the rotor 14 (and environmental enclosure 20) so as to generate a corresponding reactive magnetic field. The magnetic fields of the stator 16 and the rotor 14 interact with each other to cause the rotor 14 to rotate or cause a current into the stator windings 18 as per a conventional induction machine motor or generator operation.

A second embodiment will now be described with the aid of FIGS. 2 and 3. The second embodiment should be taken to be similar to the first embodiment described above unless otherwise specified.

The propulsion unit 201 comprises a rim driven electric machine, in which the impeller 204 serves as a rotor, while a stator 214 is accommodated within the outer casing 212. Other components of the propulsion unit shown in FIG. 3 are a series of sleeves or cans 316, 318, 320, annular core 310, supply wiring 322 for coils 324 of the stator 314, an interface component 326 for supporting the stator 314 within the casing 302, and various sealing and other components 328.

The sleeves 316, 318, 320 may be made from any suitable material. The environmental enclosure 320 may be electrically conductive but is not necessarily so. It may, for example, be made from a composite material. The inner sleeves 316, 318 need to be electrically conductive, and may be made from a metal or metal alloy such as steel or copper.

One or both (and preferably the innermost one) of the sleeves 316, 318 within the environmental enclosure 320 may be provided with apertures or slots (not shown in the Figures) which give the respective sleeve 316, 318 a configuration which, in operation, achieves a desired distribution of rotor electrical currents. For example, the respective sleeve 316, 318 could have a configuration similar to that of a squirrel cage (i.e. with axial end rings interconnected with longitudinally extending bars) which would cooperate with the annular core 310 to provide a squirrel-cage rotor effect.

In operation of an induction motor of the kind shown in FIGS. 2 and 3, current is supplied to the windings 324 of the stator 314 in a controlled manner to generate a rotating magnetic field within the stator 314. This rotating magnetic field induces current flow within the annular rotor 310, and in the sleeves 316, 318, 320 (if made of an electrically conductive material). The magnetic fields of the stator 314 and the rotor interact with each other to cause the rotor to rotate.

The electrical currents induced in the rotor are induced by the change in the stator magnetic field as this magnetic field rotates about the rotor axis. When the rotor is stationary, the change in the magnetic field is greatest and high EMFs are induced in the rotor.

Furthermore, it is known that an AC current flowing in a conductor distributes itself preferentially at the surface of the conductor in a phenomenon known as the “skin effect”. The skin effect causes the effective resistance of the conductor to increase as the frequency of the AC current increases, i.e. as the speed difference between the stator magnetic field and the rotor increases.

By using a conductive outer layer, in the form of the environmental enclosure 320, having a high electrical resistance, the skin effect is enhanced, and this results in a high torque being generated on starting of the motor.

As the rotor speed increases, the relative frequency between the stator and the rotor decreases, and the skin effect reduces. The rotor current flows preferentially in the inner sleeves 316, 318 and the core 310. Since these have a relatively low effective resistance, the machine operates at relatively high efficiency with a high power factor when running at or close to its rated speed.

By appropriate selection of the resistivity of the materials of the sleeves 316, 318 and 320, of their thickness, and of the configuration of any apertures or slots in either or both of the sleeves 316, 318, a desired characteristic for the machine can be developed in terms of the torque generated on starting, and the efficiency at normal running speed.

In a typical specific embodiment, the environmental enclosure 320 may have a resistivity of 50×10-8 to 100×10-8 Ω.m and a thickness of 0.25 mm to 0.75 mm, for example about 0.5 mm. A suitable material is stainless steel, typically having a resistivity of 72×10 8 Ω.m. Stainless steel provides a mechanically robust and corrosion resistant outer layer, while presenting the electrical system with a high resistance, particularly at start-up when high torque is achieved with a low starting current.

The outer sleeve 318 of the specific embodiment may have a resistivity of 2.5×10 8 to 3×10 8 Ω.m and a thickness of 0.75 to 1.25 mm, for example about 1 mm. A suitable material is aluminium, or an aluminium alloy. Aluminium has a resistivity of 2.5×10 8 Ω.m. The principal function of the outer sleeve 318 is to provide the ability to maintain high torque starting with a low starting current, once the rotor 310 has begun to turn. In addition, the outer sleeve 318 provides mechanical support to the environmental enclosure 320. The outer sleeve 318 may also provide damping when the motor is functioning at its rated speed.

In the specific embodiment, the inner sleeve 316 may have a resistivity below 2×10 8 Ω.m and a thickness of 1.5 mm to 2.5 mm, for example about 2 mm. The inner sleeve may be made of copper, having a resistivity of 1.68×10 8 Ω.m. The inner sleeve 316 provides the majority of the electro-mechanical work at the rated speed of the motor. Using copper, or another material with low resistivity, results in minimal losses, so increasing efficiency, and in minimal slip between the rotor 10 and the rotating magnetic filed of the stator 314.

The environmental enclosure 320 and the outer sleeve 318 may be solid, i.e. continuous about their circumference. The inner sleeve 316 may be slit in the longitudinal direction, possibly with some axial skew, over about 80% of the length of the sleeve. The end regions of the inner sleeve 316 remain continuous to short-circuit the bars left between the slits.

The annular core 310 may be in the form of a laminated ring of magnetic steel. Lamination reduces rotor iron losses, but in other embodiments the annular core 316 may be solid.

The use of metallic materials for the layers 316, 318, 320 provides a good thermal connection between the layers, improving heat dissipation.

It will be appreciated that the above specific embodiment is given by way of example only, and that other materials, resistivities and thicknesses may be employed. For example, bronze, with a resistivity of 10×10 8 to 20×10 8 Ω.m may be used, particularly for the environmental enclosure 320.

Embodiments in accordance with the invention provide electrical machines which can operate with high efficiency and a high power factor while having a rotor of relatively small radial thickness. Also, because the environmental enclosure 320 is made from a material selected primarily on the basis of its environmental protection capabilities, the present invention enables the use of a sleeve, such as the sleeve 318, disposed immediately inwardly of the environmental enclosure 320 to provide additional starting capability, so reducing the work undertaken by the environmental enclosure 320. The use of additional sleeves 316, 318 also enables the thickness of the environmental enclosure 320 to be reduced, saving weight and allowing machine optimisation.

Although it is not essential for the annular core 310 to be provided with internal rotor bars, such rotor bars, for example of circular or rectangular cross-section, could be incorporated in some embodiments. Similarly, the principles of the present invention could also be applied to permanent magnet motors in which the annular rotor 310 is provided with circumferentially distributed permanent magnets.

An alternative embodiment is shown in FIG. 4 which includes a rim drive electrical machine 410 in the form of an electrical induction machine. The electrical induction machine 410 includes a housing 412 in which a rotor 414 is located in use. The housing includes and thereby supports a single stator 416 which includes a three phase winding 418 for providing a travelling electromagnetic wave which interacts with the rotor so as to provide a torque on the rotor 414 when used as a motor, for example. The stator 416 is located adjacent the lowest point rotor with respect to vertical so as to allow the centre of gravity to be as low as possible which can be advantageous for anchoring the drive in a flow of water. The remainder of the housing 412 forms a support structure for the stators 416 and rotor 414. It will be appreciated that there may be more than one winding per phase and that the arcuate extent of the winding may be greater than shown in the Figure. It will also be appreciated that each phase may be located on a separate ferromagnetic core.

Although the invention has been described primarily in terms of an induction machine, the skilled person will appreciate that the invention is applicable to other types of electrical machines. 

1. A rim driven electrical machine for interaction with a flow of water, comprising: a rotor mounted within a housing, the rotor including at least one circumferential conductor and a central passageway, the central passageway for receiving at least one blade arrangement for rotational interaction with a fluid within the central passageway, and; a stator supported by the housing and arranged circumferentially around the rotor, the stator including windings for providing a travelling electromagnetic wave for interaction with the at least one circumferential electrical conductor of the rotor, wherein the stator extends only partially around the circumference of the rotor so as to have a first end and a second end separated by an arcuate angle.
 2. A rim driven electrical machine as claimed in claim 1, comprising a plurality of stators.
 3. A rim driven electrical machine as claimed in claim 1, wherein the circumferential ratio of stator to housing around the rotor is 1:2.
 4. A rim driven electrical machine as claimed in any of claim 1 wherein the rotor comprises an annular core of ferromagnetic material having external circumferential layers, the layers comprising an environmental enclosure, and the at least one circumferential electrical conductor is positioned between the annular core and the environmental enclosure.
 5. A rim driven electrical machine as claimed in claim 3, in which the annular core is provided with circumferentially distributed electrically conductive rotor bars.
 6. A rim driven electrical machine as claimed in claim 4, in which the annular core is provided with circumferentially distributed permanent magnets.
 7. A rim driven electrical machine as claimed in any one of claim 4, in which the environmental enclosure is made from an electrically conductive material.
 8. A rim driven electrical machine as claimed in of claim 4, in which the electrical resistance of the environmental enclosure is higher than that of the or each circumferential electrical conductor.
 9. A rim driven electrical machine as claimed in claim 1, wherein the at least one circumferential electrical conductor includes apertures which define circumferentially distributed bars extending substantially in the direction of the axis of rotation of the rotor.
 10. A rim driven electrical machine as claimed in claim 1, comprising at least two circumferential electrical conductors, wherein the or each conductor is a sleeve.
 11. A rim driven electrical machine as claimed in claim 10, in which the layers are of progressively reducing electrical resistance in the radially inwards direction.
 12. A rim driven electrical machine as claimed in claim 1, in which the sleeve immediately adjacent the environmental enclosure is radially thicker than the environmental enclosure.
 13. A rim driven electrical machine as claimed in claim 1 wherein at least a portion of the housing and rotor are separated by a substantially uniform gap which acts as a bearing surface for the rotor.
 14. A rim driven electrical machine as claimed in claim 1, in which the rotor comprises an impeller or turbine having an array of blades secured to and extending inwardly of the annular core.
 15. A rim driven electrical machine as claimed in claim 1 wherein the stator is located adjacent the lowest part of rotor when in use. 